1. Field of the invention
The present invention relates to a control system for an automatic transmission, more specifically to a hydraulic control for frictional elements incorporated into the automatic transmission when a gear shift operation is made therein.
2. Description of the prior art
An automatic transmission as conventionally incorporated into an automotive vehicle is provided with a plurality of frictional elements for performing a gear shift operation among a plurality of gear stages provided in the automatic transmission so that an optimum gear stage can be automatically established in accordance with a vehicle operating condition.
Such an automatic transmission is disclosed, for example, in U.S. Pat. No. 4,747,323 in which the frictional elements are controlled by a control unit wherein the frictional elements are actuated to be engaged or disengaged to thereby provide the optimum gear stage by means of a hydraulic pressure.
The U.S. Pat. No. 4,655,774 also discloses an automatic transmission utilizing the hydraulic pressure for controlling actions of the frictional elements for accomplishing the gear shift operation.
Each of the frictional elements is constituted by a piston actuated by the hydraulic pressure and a pair of engaging members to be brought into engagement with each other when the piston is moved by virtue of the hydraulic pressure to thereby establish an engaged condition.
However, there is a disadvantage in the conventional automatic transmission that a torque shock tends to be produced at an initial and final stage of the gear shift operation.
This torque shock in the gear shift operation is considered to be induced by a depression in a wheel rotation torque as a result of a fact that an increase of an engaging force produced in the pair of frictional members is not proportional to a displacement or stroke of the piston produced and/or an increase of the hydraulic pressure which is being applied to the piston for moving it to the engaged position.